The present invention relates to a heat-shrinkable laminated film comprising a mixed aliphatic polyamide resin layer (A) comprising aliphatic polyamide resins mixed with each other in a specific ratio and a mixed resin layer (B) comprising a saponified ethylene-vinyl acetate copolymer (hereinunder referred to as "EVOH resin") as the main constituent.
Most fatty foods such as raw meat and processed meat have irregular shapes and are not uniform in size. For packaging such foods having various shapes, a shrink packaging method using heat-shrinkable films is industrially adopted. General procedures of this method consist of the steps of inserting an object into a bag of a film, removing the air from the bag under vacuum, sealing the opening part of the bag, and heating the bag so as to heat shrink the film and bring the film into close contact with the contents, thereby obtaining a package with a fine appearance. In this method, the heat treatment also serves to sterilize the contents and is ordinarily carried out at a temperature of 70.degree. to 120.degree. C.
A film used for such shrink packaging must be excellent in its gas barrier property and adhesiveness, and be sufficiently shrunk when heated with hot water or hot air of 70.degree. to 120.degree. C.
As such a shrink packaging film, an EVOH resin film having a high gas barrier property is considered to be suitable. However, an EVOH resin film having an excellent gas barrier property is inferior in a stretchability.
To eliminate this defect, various methods have been proposed.
For example, a composite film obtained by bringing an EVOH resin film into close contact with a polyamide resin film and stretching the thus-laminated film (Japanese patent application Laid-Open (KOKAI) No. 52-115880 (1977) and, a laminated film consisting of a polyolefin layer, a polyamide resin layer and an EVOH resin layer (Japanese patent application Laid-Open (KOKAI) No. 56-136365 (1981)) have been proposed.
However, the mere lamination of an EVOH resin layer and a polyamide resin layer does not always produce a film which has satisfactory stretchability and uniform heat-shrinkability.
In addition, although the lamination of a polyamide resin layer is necessary for obtaining the required heat resistance, since the dimensional stability is deteriorated thereby, heatsetting by, for example, heat treatment is required, which may lower the workability and production yield.
Accordingly, a laminated film of a polyamide resin layer and an EVOH resin layer not only having a high gas barrier property and heat resistance but also being excellent in stretchability, heat-shrinkability and dimensional stability is now in demand.
The present inventors proposed a laminated film comprising of a mixed resin layer of a polyamide resin and an EVOH resin and a resin layer containing an EVOH resin as the main constituent in U.S. patent application No. 148,569 now U.S. Pat. No. 4,911,979.
As a result of further study undertaken by the present inventors in order to improve the stretchability, heat-shrinkability and dimensional stability of a film while maintaining the high gas barrier property of an EVOH resin and heat resistance of a polyamide resin, it has been found that when a mixed resin containing an EVOH resin as the main constituent is used in order to improve the stretchability of the EVOH resin itself, the gas barrier property and the stretchability are compatible to a certain extent, and that a laminated film having both uniform heat-shrinkability and excellent dimensional stability as well as stable processability and very improved stretchability are obtained by laminating a mixed resin layer containing an EVOH resin as the main constituent and a polyamide resin layer obtained by mixing polyamide resins having specific crystalline melting points in a specific ratio. The present invention has been achieved on the basis of this finding.